The present invention relates to small cells and, more particularly, to a cloud-based radio access network for small cells.
Small cells have become an integral component in meeting the increased demand for cellular network capacity. Cloud radio access networks (C-RAN) have been proposed as an effective means to harness the capacity benefits of small cells at reduced capital and operational expenses. With the baseband units (BBUs) separated from the radio access units (RAUs) and moved to the cloud for centralized processing, the backhaul between BBUs and RAUs forms a key component of any C-RAN.
In this work, we argue that a one-one mapping of BBUs to RAUs is highly sub-optimal, thereby calling for a functional decoupling of the BBU pool from the RAUs. Further, the backhaul architecture must be made re-configurable to allow the mapping between BBUs and RAUs to be flexible and changed dynamically so as to not just optimize RAN performance but also energy consumption in the BBU pool. Towards this end, we design and implement the first OFDMA-based C-RAN test-bed with a reconfigurable backhaul that allows 4 BBUs to connect flexibly with 4 RAUs using radio-over-fiber technology. We demonstrate the feasibility of our system over a 10 km separation between the BBU pool and RAUs. Further, real world experiments with commercial off-the-shelf WiMAX clients reveal the performance benefits of our reconfigurable backhaul in catering effectively to heterogeneous user (static and mobile clients) and traffic profiles, while also delivering energy benefits in the BBU pool.
The proliferation of mobile devices is contributing to an exponential growth of data traffic in broadband wireless networks. Sustaining these growing demands in turn requires higher spectral efficiencies from the network. Hence, operators are constantly looking for solutions that provide increased capacity without incurring significant additional capital (CAPEX) and operational (OPEX) expenses. Cloud-based radio access network (C-RAN) of small cells provides a promising solution in this direction and is being advocated both by operators (e.g., China Mobile, SoftBank) as well as service providers (e.g. LightRadio, Liquid Radio).
A C-RAN consists of three key components (FIG. 1): (i) the distributed radio access units (RAUs), each deployed with antennas at the remote site of a small-cell, (ii) a pool of baseband units (BBUs) in a datacenter cloud, run by high performance processors and real-time virtualization; and (iii) high-bandwidth, low-latency optical transport network connecting the BBUs and RAUs. The key concept of C-RAN is to separate the RAUs from the baseband processing and migrate the latter to a centralized entity. This keeps the RAUs light-weight, thereby allowing them to be deployed in large numbers for small cells. Centralized processing allows for better interference management between small cells and hence benefit from increased capacity through aggressive spectral reuse. In addition to capacity, C-RAN provides a multitude of other benefits: green infrastructure, reduced CAPEX/OPEX, easier traffic load balancing, and flexible service models.
While the BBU pool is implicitly decoupled from the RAUs in terms of physical connectivity in a C-RAN, a one-one logical mapping exists between a BBU and an active RAU. Hence, one BBU is logically assigned to generate an LTE/WiMAX frame for a given active RAU, although the mapping can change across time. We argue that such a mapping is highly sub-optimal for two reasons: (i) Generating a distinct radio signal (frame) for each small cell is important for capacity-enhancing techniques such as dynamic frequency reuse (e.g. dynamic FFR) or co-ordinated multi-point transmissions (e.g. CoMP in LTE). However, such schemes are applicable only for static users. Indeed for mobile users, for whom the problem of handovers is exacerbated in small cells, a traditional DAS (distributed antenna system) based scheme is more appropriate. In DAS, the same radio signal is transmitted to multiple small cells to provide increased coverage and diversity gain, (ii) When the traffic load is sparse in a given region, a single BBU can manage the load of multiple small cells, by serving them through a DAS. Whenever there is an opportunity to serve multiple small cells through a DAS, this reduces the number of BBUs and hence the processing (cores, DSPs, FPGAs) needed to manage a given set of RAUs, thereby resulting in energy savings in the cloud. However, allowing the C-RAN to cater to heterogeneous user (static and mobile) and traffic profiles, while also leveraging energy savings, in turn requires the backhaul to be flexible enough to support one-one as well as one-many logical mappings between BBUs and RAUs.
Towards this goal, we propose a C-RAN system with a flexible backhaul architecture, named FluidNet. While the physical optical backhaul remains unchanged, the logical connectivity between BBUs and RAUs (called overlays) is made flexible (one-one, one-many) and re-configurable to adapt to varying user profiles and traffic load conditions. We have prototyped FluidNet on a WiMAX-based C-RAN system with
4 BBUs and 4 RAUs, where the frames from the BBUs to RAUs are transported through our reconfigurable backhaul using radio-over-fiber (RoF) technology. We demonstrate the feasibility of our system over a 10 km distance between the BBU pool and RAUs. Through various real world experimental scenarios using commercial off-the-shelf WiMAX clients, we highlight the performance benefits of our reconfigurable backhaul in catering effectively to heterogeneous users (using a combination of dynamic FFR and DAS) and traffic conditions, as well as the potential for energy savings in the BBU pool.
Our contributions in this work are multi-fold.                We advocate and propose a re-configurable backhaul for C-RAN systems that can cater effectively to users of multiple profiles and varying traffic load conditions.        We prototype the first OFDMA-based C-RAN system using RoF technology.        With real-world experiments, we showcase the potential benefits of our flexible backhaul overlays with respect to both performance and energy.        